Internal combustion engines (e.g. diesel engines) typically generate an exhaust flow that contains varying amounts of particulate matter (PM). In general, particulate matter is small particles of solid matter (i.e., soot) that are suspended in exhaust gasses. The amount and size distribution of particulate matter in the exhaust flow tends to vary with engine operating conditions, such as fuel injection timing, injection volume, injection pressure, or the engine speed to load relationship. Adjustment of these conditions may be useful in reducing particulate matter emissions and average particle size in the particulate matter from the engine. Reducing particulate matter emissions from internal combustion engines is environmentally favorable. In addition, particulate matter measurements for diesel exhaust is useful for on-board (e.g., mounted on a vehicle) diagnostics of PM filters and reduction of emissions through combustion control.
On-board (in situ) sensors for particulate matter typically fall into two categories:                1. Accumulative sensors        2. Real-time sensors        
Accumulative sensors make use of the effect that exhaust particulates, especially soot particulates, tend to stick to surfaces exposed to exhaust gas. In an accumulative sensor, soot is allowed to accumulate on an exposed surface as a layer. A property of that soot layer is measured, either by vibrating the body (of which the surface is part) at its resonance frequency and then measuring the resonance frequency change that results from the accumulation of soot, or by measuring the resistance of the accumulated soot or its capacitance. When a certain threshold of the measured quantity is reached, the accumulation surface is heated to a high temperature to burn off the accumulated soot and the accumulation process starts anew. The frequency of repetition of this process is then used as a measure of the average particulate (or soot) content of the exhaust gas. Accumulative sensors are simple in construction, small enough to be installed like other exhaust sensors and relatively inexpensive. However, they have, due to their operating principle, a relatively slow response speed (not real-time) and suffer reliability issues when particles are accumulated that cannot be readily burned off.
Real-time sensors have response speeds in the millisecond range and typically make use of electrostatic effects on particulates, or use optical effects.
These sensors fall into four categories:                a. Natural charge detectors        b. Ionizing induced charge sensors        c. Contact charge sensors        d. Optical sensors        
Natural charge detectors try to detect the natural charge of particles produced during the production process. These sensors require very sensitive charge electronics and suffer from the fact that the natural charge of particles and/or their polarity can change on their way through the exhaust system.
Ionizing induced charge sensors create ions in the exhaust gas path through the sensor using a very high voltage on an electrode of a high surface curvature, like a thin wire or needle tip. Voltages are typically in the 2-15 kV range. This voltage causes a corona discharge in the particle carrying gas. Soot particles flowing through the corona discharge field acquire an electric charge. These charged particles are then collected by a collection electrode and the charge transfer rate of the collection electrode is measured. The collection electrode and the necessity to prevent charged gas ions to also transfer a charge to the collection electrode requires a fairly large and complicated apparatus that cannot be easily reduced to the size of a typical exhaust sensor.
Contact charge sensors typically use much lower voltages than ionizing induced charge sensors. In contact charge sensors soot particles coming in contact with a high voltage electrode acquire a surface charge that is determined by the surface charge density of the high voltage electrode. Typical voltages for the high voltage electrode are in the 500V to 3 kV range. These charged particles then deposit their acquired charge to grounded parts of the exhaust system or to a secondary detection electrode. The charge loss from the high voltage electrode is typically proportional to the particle concentration in the exhaust gas and is measured. However, because the resulting current (charge transfer per second) is very small, it is very difficult to isolate the high voltage electrode sufficiently to prevent current leakage. Any current that flows through an imperfect isolation to ground also creates a charge loss on the high voltage electrode and therefore causes a false sensor signal. As a way around that problem, a second collection electrode, essentially at ground voltage level, is placed in close proximity to the high voltage electrode and the charge accumulation on that electrode is measured. However, this necessitates an additional electrical connection that has to be well insulated from the high voltage supply to the electrode. For the typical temperatures encountered in the exhaust system of internal combustion engines it is very hard to find insulating materials that can withstand those temperatures and still maintain the high electrical insulating properties required.
The current detected by contact charge sensors is proportional to the particle content of the exhaust gas, but also proportional to the area of the electrodes. For the areas possible for a typically sized exhaust sensor the currents are in the low picoAmpere range for the typically encountered soot levels, which are very difficult to detect in the electrically noisy environment where internal combustion engines operate. Furthermore, the low currents require the use of electrometer grade amplifiers to detect and amplify the sensor signal. Due to limitations of current semiconductor technology, these amplifiers typically can maintain their specifications only over a narrow temperature range, which is much smaller than the typical temperature range required for vehicular applications (typically −40 to +125 degrees Celsius).
Optical sensors consist of a light source and a light detector. They measure either the opacity of the gas stream containing particles or measure light that is scattered by particles in the light path of the light source.
Common to all described real-time sensor methods is that soot accumulation on the sensor parts has a detrimental effect on the sensor performance and it is attempted to be remedied by various methods, depending on the embodiment. Either by diluting the exhaust gas with clean air, flowing compressed filtered air periodically past the electrodes (or optical parts in case of optical sensors) to blow off accumulated particles, or by heating the electrodes (or lenses in case of optical sensors) to a temperature where accumulated soot particles burn off, but not high enough to burn contacting soot particles immediately.
Further common to the described real-time sensors is that they have reaction times in the low millisecond range and can therefore for example detect changes in the soot concentration of the exhaust gas on a cylinder by cylinder basis for internal combustion engines.
Because of the above described limitations of the state-of-the-art particulate sensors for in-situ applications there is a need for a sensor that overcomes some of the limitations.
The sensor described in this invention combines certain aspects of the accumulative and the contact charge sensors in such a way as to increase the measured current by several orders of magnitude compared to a contact charge sensor, and by requiring soot accumulation on the electrodes to operate.
This allows this sensor to be scaled to the size of a typical exhaust sensor and makes it possible to use common insulators for the electrodes. In addition the sensor described in this invention does not require any special remedies to prevent soot accumulation on the sensing electrode. Compared to other real-time sensors, the sensor described in this invention has a slower response time, typically slower than 100 milliseconds, but faster than 5 seconds, which is sufficient for most applications.